Projection imaging systems are widely used in the photolithographic steps of semiconductor and printed circuit board manufacture. A conventional projection system includes a light source, a mask consisting of transmissive and blocking regions and projection optics. Conventional imaging system masks modulate light by selectively reflecting and/or absorbing the light at the blocking regions. Commonly used masks are chrome on glass and dielectric on glass.
The direct patterned ablation of workpieces by laser offers the opportunity to simplify materials processing. High power patterned light is required for direct patterned ablation.
At high powers and/or extended exposures, absorption of light at masks either erodes the mask or damages it. Chrome coated masks use reflection and absorption as the blocking mechanism while dielectric masks use reflection as the blocking mechanism. In either case, damage to the mask can occur.
Conventional photomasks consisting of chrome on quartz or low expansion glass are inadequate at high power densities because their thin (approximately 1000 .ANG.) metal layer either gradually erodes or, under more severe conditions, is rapidly removed. Reflective multilayer dielectric masks have been used successfully for a number of years in scanning ablation tools. These reflective multilayer dielectric masks can withstand single shot fluences that are relatively high (approximately 1 J/cm.sup.2 @l=248 nm). However, for sustained operation the safe operating level is at the several hundred millijoule/cm.sup.2 level. These reflective multilayer dielectric masks have nowhere near the durability of pure quartz or glass masks. In addition, the steps required to produce a dielectric mask increase cost by a factor of approximately 5.times. to that of a conventional reticule mask.